1. Field of the Invention
The present invention relates generally to wireless communication networks, and more specifically to a technique for selecting a wireless channel from multiple channels according to the interference level of the channel and maintaining the selected channel for transmission of data packets as long as the channel is of satisfactory quality. The present invention particularly relates to a method of determining the interference level of a Wireless channel of a communication network.
2. Description of the Related Art
Japanese Laid-Open Patent Application 10-66140 discloses a wireless communication network in which multiple wireless channels are shared by a plurality of subnetworks each comprising a parent host and multiple child hosts. The parent host provides management of the subnetwork and establishes packet communication with its own child hosts by sharing a single wireless channel. When the parent host of each subnetwork is powered on, it selects an idle channel from a plurality of channels allocated to the network and establishes the selected channel if there is no interference. If the upper layer of the parent host has a packet to send, it is transmitted on the established channel. On the other hand, each child host of the subnetwork, when powered on, makes a search for a channel of highest strength and selects it as the channel established by the parent host of its own subnetwork, based on the result of control packets exchanged with the parent host. If the child host detects even a single control packet on the selected channel that is transmitted from other parent host, it recognizes that there is interference and abandons the selected channel in favor of a new channel used by another parent host. Once the parent host is determined, the child host is ready to send packets on the channel established by the parent host.
A similar technique is disclosed in Japanese Laid-Open Patent Application 10-229579. According to this prior art, control packets are exchanged on a selected channel between hosts to determine its interference level. If the channel is found to be of acceptable quality, it is maintained and used for transmission of data packets. When the channel is being used for packet transmission, the channel is monitored for a channel ownership packet broadcast from other subnetwork. If such a packet is detected, it is determined that there is interference and the current channel is abandoned and a search for a new channel is initiated.
However, since the presence or absence of an interfering packet is the only factor for determining channel quality, precision measurement of interference is desired for efficient utilization of available channels. In particular, channel ownership packets broadcast from one subnetwork may be received by hosts of another subnetwork at a rate that varies with the severity of interference. Another shortcoming is that, since channel quality detection is performed independently on channel selection phase and data transmission phase based on comparison between channel quality and decision threshold, the communication between hosts may suffer from channel instability. If the decision threshold of the channel selection phase is lower than that of the data transmission phase, needs may often occur during transmission to initiate a channel search for a better channel. However, the use of lower decision threshold tends to increase the probability of lower quality channels being selected. Thus, the reselection of a channel may cause a further channel reselection.
The present invention is intended to solve these shortcomings. Prior art references which are of interest to the present invention are Japanese Laid-Open Patent Applications 8-33020 and 8-336177. In JP 8-33020, a base station collects traffic data from the network and establishes a channel to a mobile unit according to the start and end timing determined by the traffic data. Another interference detection technique disclosed in JP 8-336177 is based on the total length of time in which signals are received without error at a given rate, compared to the total length of time in which signals are received in error at the same rate.
It is therefore an object of the present invention to provide a method and a network for establishing a wireless communication channel that can avoid frequent switching of channels during packet transmission.
A further object of the present invention is to provide a method of precisely determining the interference level of a wireless channel to prevent frequent channel switching.
Briefly, the method of the present invention comprises the steps of (a) selecting a wireless channel from a plurality of wireless channels, (b) detecting interference level of the selected channel and determining therefrom a first quality value and comparing the first quality value to a first decision threshold, (c) if the first quality value is smaller than the first decision threshold, returning to the step (a) to select another channel, (d) if the first quality value is greater than the first decision threshold, using the selected channel for transmission of a packet, (e) determining a second quality value of the maintained channel and comparing the second quality value to a second decision threshold which is lower than the first decision threshold, (f) if the second quality value is smaller than the second decision threshold, returning to the step (a) to select another channel, and (g) if the second quality value is greater than the second decision threshold, maintaining the used channel and returning to the step (e).
In one aspect of the invention, the interference determination step comprises the steps of (a) broadcasting a polling packet to the network and starting a timing operation, (b) receiving a response packet from the network and incrementing a count value, (c) repeating the step (b) until the timing operation expires, (d) repeating the steps (c) to (d) a predetermined number of times each time the timing operation expires, and (e) determining the interference level of the selected channel from a ratio of the count value to the predetermined number.
In a second aspect of the invention, the interference determination step comprises the steps of (a) identifying a parent host that can be accessed from a child host via the wireless channel, (b) broadcasting a polling packet from the child host to the network and starting a timing operation, (c) receiving at the child host, a response packet from the network and incrementing a count value if the packet is received from the identified parent host or from another child host which is communicating with the identified parent host, (d) repeating the step (c) until the timing operation expires, (e) repeating the steps (b) to (d) a predetermined number of times each time the timing operation expires, and (f) deriving the interference level of the selected channel from the count value and the predetermined number.
In a third aspect of the invention in which a channel ownership packet is broadcast to the network at intervals determined by a first timer, the interference determination step comprises the steps (a) starting a second timer, (b) receiving the channel ownership packet from the network and incrementing a count value in response to the receipt of the packet and identifying a source parent host of the received packet, (c) repeating the steps (b) until the timer expires, (d) repeating the steps (a) to (c) a predetermined number of times each time the second timer expires, and (e) deriving the interference level of the channel from the count value, timeout periods of the first and second timers, a number of different source parent hosts identified by the step (b), and the predetermined number.
In a fourth aspect of the invention, the interference determining step comprises the steps of (a) starting a timer, (b) repeatedly determining the interference power level of the wireless channel until the timer expires, (c) repeating the steps (a) and (b) a predetermined number of times each time the timer expires, (d) producing a sum of the interference power levels repeatedly determined by the step (b), and (e) deriving the interference level of the selected channel from the sum, the predetermined number and a timeout period of the timer.
In a fifth aspect of the invention, the interference determination step comprises the steps of (a) starting a timer, (b) detecting interference power level of the wireless channel higher than a predetermined level and incrementing a variable by a predetermined amount in response to the detection of the higher interference power level, (c) repeating the step (b) until the timer expires, (d) repeating the steps (a) and (b) a predetermined number of times each time the timer expires, and (e) deriving the interference level of the selected channel from the incremented variable, the predetermined number and a timeout period of the timer.